The present invention generally relates to pipelines, and more particularly to systems and methods for determining the location of a pig in a pipeline based on the amount of fluid being displaced by the pig from the pipeline.
Devices known as pigs are commonly placed in and propelled through pipelines by the flow of fluid therein. The pigs may serve various functions in a pipeline, including cleaning the pipeline to ensure that the flow of fluid therethrough is not obstructed, inspecting the interior of the pipeline, and/or separating different fluids flowing through the pipeline. Pipeline pigs are also employed during a dewatering phase of pre-commissioning operations. Such dewatering may occur after the pipeline has been filled with water and pressurized to test whether it has any defects and will be capable of withstanding the pressures that will be experienced during the normal operation of the pipeline. After the testing is complete, a pig (or series of pigs) is typically placed in the pipeline and acts as a positive barrier between the water and a gas such as air that is used to displace the water from the pipeline.
It is desirable to know the location of the pig as it transits through the pipeline. If the location of the pig is not known, the pig receiver, which is located at one end of the pipeline, might be removed before the pig actually reaches the receiver. If a pig is discovered to be missing or not received, the downtime required to replace the pig receiver and displace the pig from the pipeline can be costly. Various methods have been developed for detecting the location of the pig. One such method involves using an acoustic tracking system, an acoustic signal is emitted from a device that is attached to the pig. The acoustic signal is at a pre-set frequency and this signal can be tracked as the pig transits through the pipeline. Another method is to monitor the pressure of the fluid in the pipeline and the pressure of the gas that is being used to displace the fluid from the pipeline. However, due to the compressibility of gas and other environmental factors the use of pressure measurements to determine the location of a pipeline pig is usually inaccurate. In another method, a radioactive source may be placed in the pig and a detector located outside the pipe responds to the radiation emitted by the pig as it passes a detector. Unfortunately, the radioactive source is dangerous to use and special procedures and training must be followed when handling the radioactive source. A need therefore exists for a simple, accurate method of tracking the location of a pig in a pipeline.